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Complementary resistance genes Yr73 and Yr74 (YrA) in wheat selection Avocet R confer resistance to the non-adapted barley grass stripe rust pathogen Puccinia striiformis f. sp. pseudohordei.

This is the first study on the inheritance and genetic mapping of resistance to the barley grass stripe rust pathogen (Puccinia striiformis f. sp. pseudohordeiPsph) in bread wheat. Psph, commonly infects barley grass (Hordeum leporinum, H. murinum), but about 10% of commercial barley varieties are also susceptible. We tested over 500 diverse wheat accessions and determined that less than 20% were susceptible at the seedling stage suggesting wheat is an ‘intermediate’ host to Psph. The Australian variety Teal is highly susceptible to Psph at the seedling stage, whereas selections Avocet S and Avocet R are highly resistant and resistant, respectively. We used the Teal/AvocetR doubled haploid (DH) population to characterize the resistance of Avocet R to Psph and determine whether the complementary genes Yr73 and Yr74 (YrA resistance) in Avocet R conferred resistance to Psph. Phenotypic comparison of the Teal/AvocetR DH lines in response to both Psph and Pst showed that all DH lines carrying YrA were also resistant to Psph; however, fewer DH lines were susceptible to Psph suggesting additional resistance genes. Marker-trait association analysis detected three DArT-Seq markers significantly associated with resistance to Psph, two mapping to chromosomes 3DL and 5BL in the same regions as Yr73 and Yr74 and the third mapping to chromosome 4A. Single gene stocks with the 4A gene and combinations of the 5BL and 3DL genes will be used for monitoring avirulence/virulence within Australian Psph population. Genetic analysis of seedling-susceptible T/AvR DH lines as adult plants in the greenhouse determined that Teal and Avocet R each carried at least one APR gene effective against Psph.

The University of Sydney, Plant Breeding Institute, Australia
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Association mapping for validation of genomic regions influencing adult plant resistance to leaf rust in Australian barley breeding germplasm

Elite barley breeding lines from the Australian Northern Region Barley Breeding Program were evaluated at the seedling and adult growth stages for resistance to leaf rust (LR) caused by Puccinia hordei. F3:5 lines derived from parental germplasm of different geographic origins were screened in the glasshouse and field spanning four years of trials. The 2009 and 2011 breeding populations (BP1 and BP2) comprised 360 lines and were genotyped with 3,244 polymorphic diversity arrays technology (DArT) markers. The 2012 and 2013 breeding populations (BP3 and BP4) comprised 320 lines genotyped with the DArT GBS array (DArTseq), providing 15,400 high quality polymorphic markers. Association mapping (AM) using the DArT/DArT-seq datasets and phenotypic data from 15 independent LR response assays identified a number of genomic regions associated with resistance. The BP1 and BP2 study detected a total of 15 QTL; 5 QTL co-located with catalogued LR resistance genes (Rph1, Rph3/19, Rph8/14/15, Rph20, and Rph21), 6 QTL aligned with previously reported genomic regions and 4 QTL (3 on chromosome 1H and 1 on 7H) were novel. Markers in common between the DArT and DArTseq datasets enabled integration of mapping results for LR response across the four breeding populations and all QTL detected were visualised on a single map for validation. The adult plant resistance (APR) locus Rph20 was the only region detected in all field environments. Markers and their associated sequences identified in this study will be useful for building QTL combinations involving Rph20, thereby providing stable LR resistance in improved barley cultivars. We will also highlight the advantages of AM using breeding germplasm over traditional bi-parental mapping approaches that underutilise genetic diversity and divert valuable resources into populations of low breeding value.

The University of Queensland, Queensland Alliance for Agriculture and Food Innovation, Australia
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Genetic analysis and molecular mapping of resistance in barley to the heterologous rust pathogens Puccinia graminis f. sp. avenae and P. triticina

Stem rust is considered one of the most important threats to world cereal production. The appearance and spread of the wheat stem rust pathogen [Puccinia graminis f. sp. tritici (Pgt)] race Ug99 has caused great concern for global wheat production. Barley is a host to different specialized pathogen species such as Pgt, but is characteristically a near nonhost to most non-adapted (heterologous) rust pathogens such as the wheat leaf rust pathogen [P. triticina] and oat stem rust pathogen [P. graminis f. sp. avenae (Pga)]. The barley research line SusPtrit, developed for susceptibility to heterologous rust pathogens, is a useful resource to study the genetics of nonhost resistance and to clone the genes involved, particularly due to the recent availability of the genome sequence. Studies in wheat suggest that resistance genes that are effective against multiple rust pathogens (pleiotropic) such as Lr34/Yr18/Sr55, confer durable disease control. We intercrossed the sequenced barley genotype Morex with SusPtrit to determine the inheritance of resistance to the wheat leaf rust and oat stem rust pathogens. The F2 population segregated for a single dominant resistance gene in response to both heterologous pathogens Pga and Pt. Subsequent progeny testing and genetic analysis of the segregating F3 population will be performed to map and determine the relationship between the resistance genes. Large F2 populations were developed to fine map and clone the genes, and ultimately to transfer them into related crop species as an alternative approach for crop protection.

The University of Sydney, Plant Breeding Institute, Australia
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Sources of resistance to stem rust in the USDA Barley iCore Collection

Stem rust is an important disease of wheat and barley. Barley is genetically vulnerable to stem rust with few identified resistance genes and sources. Stem rust resistance breeding is largely based on the gene Rpg1, first incorporated into North American barley cultivars in the 1940s. To identify potentially new resistance sources, the USDA-ARS National Small Grains Barley iCore Collection (1,860 accessions) was evaluated for reaction to stem rust at the seedling and adult stages. The adult stage evaluations were conducted at Njoro, Kenya (race TTKSK/TTKST), and St. Paul, Minnesota (race QCCJB), for two seasons, and the seedling tests (race TTKSK) in the BSL-3 greenhouse at St. Paul, Minnesota. At St. Paul, between 7 and 10% (132-203) of the accessions exhibited resistance, whereas in Kenya, 11-14% (218-261) were resistant. Correlation between years was higher in Kenya (0.60) than it was at St. Paul (0.48). Approximately 15% (277) of the collection gave moderately low to low reactions to TTKSK at the seedling stage. From these initial tests, 290 accessions were chosen based on diversity of reaction, origin of plant material, or stability across environments. These accessions were then further evaluated with a suite of races at the seedling stage to postulate resistance genes. Of these selections, 244 gave reactions suggesting they carry adult plant resistance. The remaining 46 accessions gave low to very low reactions to one or more races. Based on country of origin and resistance spectrum 15 accessions were predicted to have the rpg4/Rpg5 complex, including a subset from Switzerland. The remaining 31 have a reaction spectrum, country of origin or pedigree that does not suggest the presence of the rpg4/Rpg5 complex. Molecular tests will be used to confirm the presence of this complex in these materials. 

Department of Plant Pathology, University of Minnesota, USA
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Cytogenetic analysis and molecular mapping of inheritance of resistance in barley to Puccinia striiformis

Plants are generally non-hosts to most diseases. Barley is a host to Puccinia striiformis f. sp. hordei, but is a near non-host to P. striiformis f. sp. tritici (Pst) and to P. striiformis f. sp. pseudohordei (Psp), which cause stripe rust on wheat and barley grass (Hordeum murinum, H. leporinum), respectively. This study was carried out to determine the inheritance of resistance in barley line 81882/BS1 using the mapping population: 81882/BS1/Biosaline-19. 81882/BS1 is a H. vulgare derivative of cv. Vada, carrying an introgression from H. bulbosum on chromosome 2HS, and Biosaline-19 is susceptible to both Pst and Psp. Phenotyping of F3 lines with Psp culture 981549 and Pst pathotype 134 E16 A+ showed that 81882/BS1 carried two genes for resistance to Psp, and three genes for resistance to Pst. Cytogenetic analysis and molecular mapping were performed to further characterize the resistance of 81882/BS1 to Psp. Joint phenotypic and cytogenetic analysis indicated that at least one of the genes for resistance to Psp was associated with the H. bulbosum introgression previously located on chromosome 2H (Zhang unpublished). Preliminary molecular mapping of 15 non-segregating resistant and 15 non-segregating susceptible lines using >10K DArTseq molecular markers located the second gene on chromosome 1H. This gene was probably contributed by Vada. Further studies are underway to confirm the locations of these two loci by fine mapping.

The University of Sydney, Plant Breeding Institute, Australia
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